The present invention relates to an improvement of an automatic document feeder which is provided to an image forming apparatus such as an electrophotographic copying machine.
An automatic document feeder (ADF) is generally provided on a platen glass of an image exposure section of a copying machine or the like. In an optical system travelling type copying machine provided with the automatic document feeder, documents which are fed one by one from a document stacking tray (a stack section) on which documents are stacked, by the automatic document feeder, are fed to a platen glass by a conveyance belt, exposed by an exposure lamp provided in the copying machine main body, and then conveyed onto a document delivery tray.
A conventional exchanging operation of a document located on a platen glass by the automatic document feeder is as follows. After a document delivery sensor detects that a document on a platen has been delivered, a subsequent document is fed from the stack section to the platen glass and stopped at a predetermined position. Therefore, a long document exchange time is necessary. Accordingly, a long copy-processing time is necessary, so that copying productivity is lowered.
When a document is automatically fed to and delivered from the platen using the automatic document feeder, and the document is exchanged within the returning time of the optical scanning system of the copying machine main body, the copying productivity is 100%, and long copy-processing time is not necessary. For this purpose, it is necessary that speed of the document conveyance from a document double feeding protection position to a document stop position on the platen glass is increased. However, when the document conveyance speed is increased, the following disadvantages are generated: a large motor is necessary; the consumed power is increased; noises are generated; a conveyance belt is worn down; the degree of document damage is increased; and the document separation property is lowered.
In order to solve the foregoing problems, conventional technology of an automatic document feeder by which the document exchange time is decreased without increasing the document conveyance speed, is disclosed in Japanese Patent Publication Open to Public Inspection No. 236136/1989.
The operation of the document feeder is as follows: a first document is fed to a predetermined position on the platen glass; successively, a second document is fed to an upstream position, which is not on the platen glass, adjoining the predetermined position; next, the first document, the image of which has been read out, is conveyed to a position to which the document can be conveyed and which is downstream adjoining the predetermined position on the platen glass, by a conveyance means; at the same time, the second document is conveyed to the predetermined position on the platen glass; and for a third document and documents after that, the foregoing operations are repeated and the document is successively conveyed and stopped. (A flying document feeding method).
However, in the foregoing document feeding method, the existence of the subsequent document can not be confirmed until the preceding document has been completely fed out. Therefore, when the number of copying sheets is small, a recording sheet feeding path is long, and a recording sheet is fed after the existence of the subsequent document has been confirmed, the speed of copying operation is practically decreased.
Further, in a document feeding method of the copying machine disclosed in Japanese Patent Publication Open to Public Inspection No. 140364/1985, this method is a three document simultaneous feeding method, and when documents which are smaller than a certain size, are continuously copied, the copying speed can be increased. In this case also, the existence of the subsequent document can not be confirmed until the preceding document has been fed out.
Further, in the copying machine equipped with the automatic document feeder, in the case of a side loading type sheet feeding means in which a sheet feeding cassette is provided to the side of the copying machine main body, the distance from a leading edge of the sheet feeding cassette to a registration roller of a feeding apparatus of the copying machine main body is comparatively close. Therefore, the time from the generation of a document size detection signal of documents stacked on a stacking tray in the automatic document feeder to the time at which a recording sheet is taken out from the corresponding sheet feeding cassette and conveyed to the registration roller, is approximate to the time in which the document is conveyed to a stop position on the platen glass. Therefore, when documents with a plurality of sizes are stacked together, it is not necessary to particularly reduce the document exchanging time.
However, in a recording sheet feeding apparatus in which the distance from a feeding start position (a leading edge of a recording sheet) from a sheet feeding cassette to a registration roller is large, for example, in a front loading multiple stage sheet feeding cassette type (a front loading type as shown in FIG. 1) recording sheet feeding apparatus, or a large capacity sheet feeding tray type (LCT) recording sheet feeding apparatus, recording sheet feeding time, in which a recording sheet fed from a sheet feeding cassette (or a sheet feeding tray) is conveyed to a registration roller, is increased. Therefore, even when the subsequent document arrives at a predetermined position on the platen glass, a corresponding recording sheet does not arrive at a predetermined position in an image forming section, so that the copying productivity={the number of copied sheets per minute (CPM) / the number of documents processed per minute (OPM)} is decreased. Especially, in a high performance copying machine in which an image formation processing speed and a document conveyance speed are increased and documents can be processed at high speed, the delay of the foregoing recording sheet feeding is a problem.
Further, in a copying machine in which the subsequent document is fed to an intermediate document feeding path from a document stacking tray provided in the document feeding position to a platen glass, and the document stands-by there for the next processing, the subsequent document size can be detected on the way to the intermediate path, and therefore the subsequent recording sheet corresponding to the size can be fed previously. However in this type of apparatus, a document feeding path is long, the size of the apparatus is large, the document can not be fed satisfactorily, or document jamming can hardly be processed, which is a problem.